As we mentioned on other articles, the first place we like to take a look when opening a power supply to have a hint about its quality is its filtering stage. The recommend components for this stage are two ferrite coils, two ceramic capacitors (Y capacitors, usually blue), one metalized polyester capacitor (X capacitor) and one MOV (Metal-Oxide Varistor). Very low-end power supplies use fewer components than that, usually removing the MOV, which is essential for cutting spikes coming from the power grid, and the first coil.

In this stage this power supply is flawless, as it has more than the minimum recommended number of components for this stage: two extra metalized polyester capacitors (X capacitors), two extra ceramic disc capacitors (Y capacitors) and two extra ferrite coils, see Figures 7 and 8.

In our first analysis we posted that this power supply didn't have a MOV. However a friendly technician that works at one Seventeam distributor pointed out to us where the MOV’s are located: they are squeezed between the two electrolytic capacitors from the voltage doubler (the two big capacitors located on the primary) and connected after the rectifying bridge, and not before this component as usual – and that is why we didn’t find them at the first time. My sincere apologies to our readers for my lack of attention.

click to enlarge
Figure 7: Transient filtering stage (part 1).

click to enlarge
Figure 8: Transient filtering stage (part 2).

A very interesting feature from this power supply is that its fuse is inside a fireproof rubber protection. So this protection will prevent the spark produced on the minute the fuse is blown from setting the power supply on fire.

Now let’s have a more detailed discussion on the components used on Seventeam ST-420BKV.